French authorities have thwarted a possible attack using either highly toxic ricin gas or explosives, the country’s Interior Minister Gerard Collomb has said. Just a few grains of ricin powder is enough to kill an adult human.

Two men of Egyptian origin were arrested in connection with a foiled attack using powerful ricin poison or explosives, Collomb told French BFMTV on Friday. The arrested men, who are brothers, had tutorials showing how to build a ricin-based poison, the minister said.

Ricin is a naturally occurring toxic protein, which is especially harmful its gas form. The lethal dose for an average adult is less than 2 milligrams if the exposure is from injection or inhalation.